1. Field of the Invention
The present invention relates to a hydraulic shock absorber for use in, for example, a suspension system of an automobile.
2. Description of the Prior Art
A conventional mechanism for generating damping force will first be explained with reference to FIGS. 1 to 3 which show in combination a typical example of a conventional hydraulic shock absorber.
An outer casing 1 is tightly closed at the upper and lower ends thereof by means of upper and lower covers 2 and 3, and a cylinder 4 is provided inside the outer casing 1. A piston 5 is slidably received in the cylinder 4 so that the piston 5 is reciprocatable. A piston rod 6 is connected to one end of the piston 5. The other end of the piston rod 6 extends through the upper cover 2 and projects outside the outer casing 1. At the mutual upper end of the outer casing 1 and the cylinder 4 are provided a rod guide 7 for guiding the rod 6 and a seal member 8 for hermetically sealing the inside of the cylinder 4.
The following is a description of a conventional valve mechanism 9 which is provided on the piston 5 to generate damping force.
In general, the valve mechanism 9 of this type comprises a plurality of flexible disk valves 10 which are laid one on top of another, a valve seat 11 on which the stack of disk valves 10 rests, and a combination of a valve retainer 12 and a spring member 13 for biasing the disk valves 10 toward the valve seat 11. Two disk valves 15 and 16 which are disposed underneath the stack of disk valves 10 are arranged to define a fixed orifice 14. The upper disk valve 15 is provided with a U-shaped notch 15a which extends from the outer periphery toward the axial center thereof, while the lower disk valve 16 is formed in the shape of a ring.
A gap 17 which is formed between the disk valves 10, 15 and 16 when laid one on top of another is defined as an opening of the orifice 14.
The function of the valve mechanism 9 will next be explained. As the hydraulic shock absorber extends, oil flows from an oil chamber A defined at the upper side of the piston 5 to an oil chamber B defined at the lower side of the piston 5. At this time, when the speed of movement of the piston 5 is relatively slow, damping force is generated by virtue of the throttling function of the fixed orifice 14, whereas, when the speed of the piston 5 is relatively fast, the inner peripheral portion of the stack of disk valves 10, 15 and 16 is bent so as to generate damping force.
The above-described prior art arrangement suffers, however, from the following problems. With the valve mechanism 9, when the speed of the piston 5 is relatively slow, that is, before the inner peripheral portion of the stack of disk valves 10 is bent, damping force is provided by the fixed orifice 14 alone and the change of the damping force in the relatively slow piston speed region takes the form of a quadratic curve such as that shown in FIG. 3. Therefore, if the level of damping force at the piston speed immediately before the inner peripheral portion of the stack of valves 10 is bent, e.g., at the point A, is set at an optimal value, the level of damping force at the point B at which the piston speed is lower than that at the point A is below the desired value, so that the conventional hydraulic shock absorber cannot exhibit its function to the full in this piston speed region.